Digital receivers are a necessary part of many communication systems. They are characterized by several parameters, one being their sensitivity. Sensitivity is a measure of how weak an incoming signal can be and still be detected with an error rate less than a predetermined value. Errors are caused by “noise” that is introduced in the transmission process and/or introduced in the receiver itself where amplifier noise or pickup is the case. Additionally, in a multi-signal configuration, there may be noise-like artifacts due to interference in a given channel, as the result of the presence of other signals in adjacent channels. Over long distances, and possibly with many amplifiers, the signals may become distorted due to dispersion and due to nonlinearities. These distortions may also worsen the receiver's BER for a given signal strength.
Receiver sensitivity is important inasmuch as a higher sensitivity for a specific receiver may provide for greater maximum distances of operation. For example in optical fiber-based communication systems, a 2 dB improvement in receiver sensitivity may lead to an increase in usable fiber lengths of 5-10 km. In the case of long haul systems with optical amplifiers placed periodically along the length of the fiber, the spacing can be increased between amplifiers by a similar amount.
Traditionally, receiver sensitivity is improved by carefully constructing a linear filter that maximizes the SNR prior to detection. Specifically, for a baseband signal, where the signal occupies a frequency spectrum of DC to about half the baud rate, a low pass filter with shape similar to the signal will be used so that any noise whose spectral components lie above the cut-off frequency are largely removed.
However, the use of a conventional filter is found to limit the bit error rate (BER) that may be achieved, since there is a trade-off between the bandwidth of the filter and amount of noise that is acceptable for a given application.